1. Field of the Invention
This invention relates to an optical device for recording and reading a data carrier.
2. Description of the Prior Art
The invention is more particularly concerned with recording and reading systems in which the data carrier is a disk. It is possible at the present time to record approximately 10.sup.10 binary data elements on a disk of about thirty centimeters in diameter. These systems can be employed as mass memories for digital data-processing assemblies. Systems of this type must permit random access to any predetermined portion of the recorded data such as, for example, a block of binary words of either fixed or variable length. Furthermore, the computation rate of the data-processing units calls for a large number of transfers between peripheral memories and the computation units. In consequence, access to any predetermined track of a movable data carrier must necessarily take place within the shortest possible time, both in order to read prerecorded data and in order to record said data irrespective of the position of said track on the disk. Typically, the mean access time must be less than 100 milliseconds.
Single-track access devices of currently available types are essentially intended for disks on which the recorded information contains a video signal in digital form. In these systems, the data access time is of the order of a few seconds, which is sufficient for this application.
In data access devices of the prior art, correct radial positioning of a optical recording and/or reading head is carried out by mechanical means either for producing displacements of the head or for producing displacements of the disk (as is more frequently the case). When the reading head and the disk are correctly positioned with respect to each other, radial following of the circular or helical track on which the information has been (or is intended to be) recorded is carried out by means of a galvanometer mirror which is capable of moving about an axis parallel to the plane of the disk and reflects a beam produced by at least one source of radiant energy comprising a laser. The head is also provided with a device for controlling vertical displacements of the objective employed for recording and reading. In actual practice, provision is made in the majority of instances for two sources, namely one source for reading and one source for recording.
Irrespective of the arrangements adopted, the mass of the moving system is too heavy to be compatible with the mean access times which are desirable in the field of data-processing. By way of example, in the method which consists in displacing a disk coupled to a rotary drive mechanism comprising in particular the driving motor, the mass to be set in motion is of the order of 1 Kg. In order to reduce the access time, it is possible to make use of a device in which only the recording and reading head is capable of moving whilst the laser-type energy source remains stationary. By way of example, the device comprises optical means of the afocal type for providing an optical coupling between the light energy source and the recording and reading head.
In a device of this type, provision is made for a moving system comprising a galvanometer mirror and a focusing objective which is movable with respect to the data carrier, and two stationary radiant-energy sources each comprising a laser emitter. The optical magnification means of the afocal type are inserted between the moving system and the radiant energy sources. The magnification is sufficient to ensure that the beam emerging from the optical means covers the entire entrance pupil of the objective. By virtue of this arrangement, the beam which reaches the moving system remains a beam of parallel rays irrespective of the position of said moving system with respect to the radiant energy sources. Reading and control of recording are carried out by detecting the intensity of the beams reflected from the surface of the read or recorded zones.
The device which has just been described in wholly suited to the application of gas lasers. This does not hold true in the case of sources of the semiconductor laser type which have been introduced more recently. These lasers entail the need for a collimating optical system and are equivalent to a source having a large useful emission diameter. It is no longer possible to use an afocal lens. The two parallel beams for reading and recording respectively which are emitted by the two corresponding sources and combined to form a composite beam have to be inclined at only a very small angle with respect to each other. It therefore becomes very difficult to detect these two beams separately in devices of small overall size. Long distances are in fact necessary in order to obtain complete separation of the beams.
The invention proposes a recording and reading device which remains compact, which permits perfect separation of the reflected recording and reading beams, and still has satisfactory power efficiency.